Gas barrier films designed to block active gases are one of the most important topics in many application areas such as solar cells and electronic devices, as well as food packaging. In general, in order to prepare a gas barrier film, aluminum is vacuum deposited on a polymer film substrate (PET, OPP, CPP, LDPE, etc.), or methods of depositing or coating with an inorganic material such as silica or alumina, and of depositing a polymer film having excellent gas barrier properties such as ethylene vinyl alcohol copolymer (EVOH), nylon, polyacrylonitrile, and polyvinylidene chloride (PVDC) are used. However, in the case of an aluminum deposited film, it becomes opaque; in the case of depositing an inorganic material, there is a disadvantage in that the production cost is increased due to a high deposition cost; and in the case of a polymer film with gas barrier properties, there is a problem that it is difficult to form a thin film.
Graphene has attracted much attention in recent years due to its high gas barrier function, as well as its mechanical, thermal, and electrical properties. Since graphene itself has gas barrier properties, it was expected that the above object could be achieved by using a graphene thin film or a graphene nanoplatelet formed through molecular deposition, but in terms of manufacturing costs and due to a low gas barrier performance, there are restrictions on commercial use.
Large-area graphene synthesized by deposition is considered as an ideal gas barrier material due to its high transparency and small pore diameter, which is smaller than the dynamic diameters of various gases. However, due to molecular structural defects that result from more or fewer carbon atoms besides benzene rings formed in the actual synthesis process, and due to physical defects such as tearing which may occur during the adhesion process of a synthesized large-area graphene film to a substrate, the gas barrier properties are reduced. Further, as the area to be formed becomes wider, expensive equipment and facilities corresponding to the wider area are necessary, and there are limitations to industrial manufacture such as increased manufacturing process cost, etc. Therefore, a method for synthesizing large-area graphene without defects and its application as a product are still difficult to achieve for these reasons, and there is a strong demand for a barrier film that can be easily industrially prepared as a replacement therefor.
In order to prepare a gas barrier film using graphene nanoplatelets exfoliated from graphite, graphene is mixed with a polymer to form a film, or graphene is modified so that it can be easily dispersed in a solvent by converting to graphene oxide (GO) having a hydroxyl, an epoxide, a carbonyl, and/or a carboxyl group, and the dispersion thereof is used as a coating solution.
In the case of the graphene nanoplatelet-polymer composite film, it forms ‘tortuous channels’ having a nano-barrier effect by a graphene nanoplatelet, which is dispersed in the polymer and has gas barrier properties, thereby forming a relatively long elongated gas diffusion path, which exhibits a reduction in the oxygen transmission rate (OTR). However, the content of the graphene nanoplatelet added in a polymer substrate for preparing a barrier film cannot be increased above a certain level, and the reduction rate of the oxygen transmission rate that can be achieved is limited due to the low dispersibility of the graphene nanoplatelets in the polymer substrate.
On the other hand, in the case of using a graphene oxide nanoplatelet with improved dispersibility with respect to the solvent, gaps between layered nanoplatelets exist, and as gas can permeate through the gaps, it is difficult to achieve gas barrier properties as expected, unless coated very thickly.